1. Field of the Invention
The invention relates to a treatment method for an optical fiber.
2. Description of Related Art
After rendering optical fibers into a cable and laying the optical fiber cable, the transmission loss in a wavelength band of about 1400 nm of the optical fiber increases, and the transmission characteristics of the optical fiber degrade.
In order to inhibit this degradation of the optical fiber transmission characteristics, a treatment method for optical fibers is applied in which an optical fiber is exposed to hydrogen or deuterium in advance.
However, when carrying out a treatment method in which the optical fiber is exposed to hydrogen (hydrogen treatment), there is a problem in that the initial transmission loss of the optical fiber in a wavelength region of about 1383 nm increases. In contrast, when carrying out a treatment method in which the optical fiber is exposed to deuterium (deuterium treatment), no such problem occurs. Thus, the hydrogen treatment or the deuterium treatment must be selected and applied depending on the object.
Various methods have been proposed for applying a deuterium treatment to an optical fiber. In one method (see, for example, Japanese Unexamined Patent Application, First Publication No. 2003-137580), a gas that contains deuterium gas (hereinbelow, referred to as a “deuterium containing gas”) continuously flows into a sealable treatment chamber, and the atmosphere in the treatment chamber is replaced by the deuterium containing gas. In another method (see, for example, Japanese Unexamined Patent Application, First Publication No. 2004-226979), after decreasing the pressure inside the treatment chamber, the inside of the treatment chamber is filled with the deuterium containing gas, and thereby the atmosphere inside the treatment chamber is replaced by the deuterium containing gas. In the latter method, recovery of the deuterium containing gas is possible.
In addition, the hydrogen test method stipulated in IEC60793-2-50 is an example of a method for applying a hydrogen treatment to an optical fiber. In this hydrogen test method, an optical fiber is exposed to 1% hydrogen gas atmosphere or 100% hydrogen gas atmosphere at room temperature until the transmission loss at a wavelength of 1240 nm is equal to or greater than 0.03 dB/km. However, in this hydrogen test method, specific conditions of the treatment methods (treatment conditions) are not stipulated.
In the case in which a deuterium treatment is applied to an optical fiber, when using the method in which the deuterium containing gas continuously flows into the treatment chamber, the treatment is applied while eliminating deuterium containing gas until the inside of the treatment chamber reaches a predetermined deuterium concentration. Thus, each time the deuterium treatment is applied, it is possible to maintain a predetermined deuterium concentration in the treatment chamber.
However, in order to increase the production efficiency, the treatment must be applied at one time to an optical fiber having a long length, and accompanying this, the volume of the treatment chamber for applying the treatment to the optical fiber becomes large. Depending on how large the volume of the treatment chamber becomes, the amount of deuterium containing gas necessary for replacing the atmosphere in the treatment chamber increases, and thereby the cost also increases.
In contrast, when using the method in which the treatment chamber is filled with deuterium containing gas after the pressure in the treatment chamber has been reduced, because creating a complete vacuum inside the treatment chamber is difficult, air that remains in the treatment chamber from each treatment mixes with the deuterium containing gas. As the number of treatments increases, the deuterium concentration in the recovered deuterium containing gas becomes low.
When the deuterium concentration in the deuterium containing gas becomes low, there are also concerns in that not only is a long time required until the deuterium has sufficiently permeated the optical fiber, but also that the deuterium treatment may become inadequate. Thus, in order to prevent defective deuterium treatments, it is necessary to monitor the deuterium concentration when applying the deuterium treatment to the optical fiber.
An inflammable gas detector, an optical gas densitometer, or the like are examples of devices that measure the deuterium concentration. When using an air-based deuterium containing gas, it is possible to measure the deuterium concentration by such measuring devices. Note that here, any gas whose composition ratios are similar to those of the atmosphere is referred to as “air”.
However, when air is mixed into the deuterium containing gas and the composition ratios of the gas differ from those of the atmosphere, it becomes impossible to measure the deuterium concentration correctly by using the measuring devices described above. In a combustion densitometer such as an inflammable gas detector, deuterium is combusted, and the deuterium concentration is calculated from the amount of heat generated during the combustion. Thus, in the case in which, instead of air, an inert gas such as nitrogen, argon, helium, or the like serves as the base, the combustion becomes poor, and thus the concentration cannot be correctly measured.
In addition, in an optical densitometer such as an optical gas densitometer, the deuterium concentration is calculated from the relative index of refraction of the deuterium containing gas. Thus, in the case in which the composition ratio of the gas that serves as the base fluctuates, it is not possible to measure the deuterium concentration correctly.
In addition, it is possible to measure the deuterium concentration by analyzing the molecules of the deuterium gas included in the deuterium containing gas. However, this is not preferable because the measurement takes time, and thus the production costs drastically increase.